1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a semiconductor device and a method of manufacturing the same, which are suitably used when MOS transistors having different threshold voltages ar to be obtained by a single step.
2. Description of the Related Art
Conventionally, the threshold voltage of a MOS transistor and, more particularly, of a D-type (Depletion) transistor, is determined by the type and amount of an impurity used for the step of ion implantation in a channel, as shown in FIGS. 1A to 1C. FIG. 1A shows a step of forming a gate insulating film 2 having a thickness of, e.g., 200 A on, e.g., a p-type semiconductor substrate 1. FIG. 1B shows a step of ion implantation in a channel for controlling the threshold voltage of a MOS transistor. In FIG. 1B, phosphorus ions 3 are implanted at an acceleration energy of 70 KeV to a concentration of about 1.times.10.sup.12 cm.sup.-2 in order to form, e.g., a depletion-type transistor. FIG. 1C shows a step of forming a gate electrode 4 and source and drain diffusion layers 5. In this case, when n-type polysilicon is used as the material of the gate electrode 4, the threshold voltage of the transistor is about -2 V. Reference numeral 6 denotes an insulating film; and 7, an aluminum wiring layer.
In this conventional example, however, when transistors having different threshold voltages are to be formed, different types of ions must be implanted in different amounts at different places in order to set different threshold voltages. In other words, the same number of photoengraving steps as that of the required threshold voltages are required, resulting in a great increase in the number of steps. A conventional well layer is obtained by performing impurity implantation through a considerably large opening of a mask pattern (opening diameter). Therefore, the conventional well have a uniform surface impurity concentration. To form transistors having different threshold voltages in the conventional well, it is also necessary to implant different types of ions in different amounts into the different parts of the well. In this case, too, the same number of photoengraving steps as that of the threshold voltages need to be performed.